1. Field of the Invention
The present invention relates to driver circuits for light emitting diodes (LEDs) and in particular to driver circuits which provide voltage controlled dimming. Still more particularly, the present invention relates to a driver circuit providing enhanced trim resolution in voltage controlled dimming of light emitting diodes.
2. Description of the Prior Art
Like many other display systems, aircraft instrumentation displays frequently employ illuminated indicators. Originally incandescent bulbs were employed for this purpose. However, a variety of factors have motivated replacement of incandescent bulbs with light emitting diodes (LEDs) in such applications, including improvements in power consumption, heat generation, and operating lifetime.
Standards exist for the luminance, or brightness level, of illumination sources in aircraft instrumentation displays. Generally, for example, the luminance required for a sunlight-readable indicator must be in the range of at least 300-500 foot-lamberts. Similarly, the accepted indicator luminance for commercial night-flying is approximately 15-20 foot-lamberts; for military night-flying, approximately 1 foot-lambert; and for night vision imaging system (NVIS) compatible flying, approximately 0.1 foot-lambert. Additionally, for voltage-controlled dimming of illuminated indicators, the various voltage levels at which these luminance ranges are achieved (e.g., sunlight readable illumination at approximately 28 volts and commercial night flying illumination at approximately 14-15 volts) have also become effectively standardized by industry expectations, since aircraft instrumentation designers would prefer to utilize existing analog circuitry in implementing voltage-controlled dimming of illuminated indicators.
These luminance standards and corresponding voltage levels are based on incandescent bulbs as illumination sources within aircraft displays. However, LEDs have different luminance-power characteristics than incandescent bulbs. When replacing incandescent bulbs with LEDs in aircraft instrumentation, or when employing controls designed for incandescent bulbs with LEDs, alteration is required of the luminance-power characteristics of LEDs to satisfy the luminance standards and corresponding voltage level expectations.
One approach to satisfying the luminance standards and voltage level expectations when utilizing LEDs involves providing a mechanism for compensating for changing the portion of the applied input power which is actually transmitted to the LEDs. The portion of the applied input power which is transmitted to the LEDs changes across the operating range of input power to the LED illuminated indicator, matching the power transmitted to the LEDs to the power which is required by the LEDs to achieve approximately the same luminance as an incandescent bulb receiving the same input power. However, this approach negates at least some of the reduction in power consumption achieved by employing LEDs in lieu of incandescent bulbs.
An additional problem in satisfying the luminance standards and voltage level expectations with voltage controlled dimming of LEDs arises from the inherent luminance-power power characteristics of series-connected LEDs. Parallel-connected LEDs may be employed to increase luminance, but require proportionally more power than a single LED. Series-connected LEDs, although requiring less additional power over a single LED than is required by parallel-connected LEDs, have inherent voltage-luminance characteristics which are poorly suited to voltage-controlled dimming, as illustrated in FIG. 6. Curves 602a and 602b within FIG. 6 are voltage-luminance plots for two common types of incandescent bulbs. These curves provide significant changes in luminance across the operating range of input voltages as the applied input voltage is brought down from 28 volts to the minimum voltage required for illumination (about 2.5 volts).
Curves 604, 606, 608, 610 and 612 are voltage-luminance plots for, respectively, one LED, two series-connected LEDs, three series-connected LEDs, four series-connected LEDs, and eight series-connected LEDs, all with a series resistor to produce 300-500 foot lamberts at 28 volts. As can be seen from curve 604, for example, the luminance of a single LED falls off very gradually as the applied input voltage is brought down from 28 volts to about 6 volts, after which the luminance falls off rather sharply up to the turn-on voltage for the LED. This provides poor trim characteristics for voltage-controlled dimming of the LED, offering only a narrow range of applied input voltages within which significant changes in luminance are achieved. This limitation is exacerbated by the addition of more LEDs within a series-connection, which increases the turn-on voltage for the LEDs. As a result, curve 612, representing eight series-connected LEDs, provides extremely poor trim resolution for voltage-controlled dimming, and only within the upper half of a 28 volt operating range.
It would be desirable, therefore, to improve the trim characteristics of an LED illumination source to provide improved responsiveness of luminance to voltage changes across the entire operating range of applied input voltages. It would further be advantageous to employ series-connected LEDs for power savings and for improved trim characteristics.